Inkjet printing apparatuses can be used for single-color or multicolor printing of a printing substrate, for example a belt-shaped recording material made of the most varied materials (paper, for example). The design of such inkjet printing apparatuses is known; see for example EP 0 788 882 B1. Inkjet printing apparatuses that operate according to the Drop on Demand (DoD) principle have a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles—controlled by a print controller—excite ink droplets in the direction of the printing substrate web, which droplets are directed towards the printing substrate in order to apply print dots there for a print image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
Given printing of a printing substrate web, it is sometimes necessary to stop the printing substrate web in a pause function during the printing operation, for example in order to monitor the register quality after printing a print job, or in order to correct problems in the post-processing of the printing substrate web. After the resumption of the printing substrate web, print image disruptions can then occur at those web segments that were located directly under the print heads after activation of the pause function. Due to the relatively large transfer printing zone in inkjet printing apparatuses (inkjet printing systems, for example), in particular in color printing, the print image disruptions created due to the pause correspondingly cause a great deal of maculature. The occurring print image defects contain print image distortions, color register errors and trapezoidal print image distortions. The causes for these are the swelling or shrinking of the printing substrate web during the pause, and the position shifts of the printing substrate web below the print heads that are linked with this.
These problems are explained using prior art FIG. 1. A printing unit 1 and a printer controller 2 from a printing apparatus DR are shown. The printing unit 1, which has print bars 4 with print heads 5 in series (as viewed in the transport direction of the printing substrate web 3) is arranged along a printing substrate web 3. In color printing, for example, one print bar 4 is respectively provided per color to be printed. The printing substrate web 3 is moved past the print bars 4 with the aid of a discharge roller 9; it is thereby placed on a saddle with guide rollers 8. Arranged at the input of the printing unit 1 is a rotary encoder wheel 6 that is driven by the printing substrate web 3 and that generates rotary encoder pulses depending on the feed motion of the printing substrate web 3, which rotary encoder pulses are supplied to the printer controller 2 and are used by the print controller 2 in order to establish the point in time of the initiation of the print process at the individual print heads 5. The printing substrate web 3 is supplied to the rotary encoder wheel 6 by a drive roller 7 arranged before the rotary encoder wheel 6.
In FIG. 1 it is now shown in principle how the printing substrate web 3 can be affected in the individual web segments BA through the printing apparatus DR by the printing unit 1 or the environmental air, for example given a standstill of the printing apparatus DR. In the web segment BA1 between drive roller 7 and rotary encoder wheel 6, the print substrate web 3 is exposed to environmental air, with the consequence that here a swelling of the print substrate web 3 can occur due to the humidity of the environmental air. The change to the print substrate web 3 that is caused by this in the longitudinal direction is compensated with the aid of the rotary encoder wheel 6, however. In the web segment BA2 after the rotary encoder wheel 6 up to the printing unit 1, a swelling of the print substrate web 3 due to the environmental air can likewise occur which, however, remains unaccounted for by the rotary encoder wheel 6. This also applies to the web segment BA3 under the print heads 5 of the printing unit 1; there the print substrate web 3 can shrink due to the operating temperature of the print heads 5; however the printing substrate web 3 is also exposed to the environmental air, such that the web segment BA3 can swell due to the moisture in the environmental air, in particular given greater distances between the print bars 4. The two influences overlap. The printing substrate web 3 is thus exposed to different environmental influences from the drive roller 7 up to the discharge roller 9, which different environmental influences can lead to a shrinking or to a swelling of the printing substrate web 3. This can lead to the print image errors mentioned above, in particular if the printing process is started again after a pause during the print operation.
Given triggering of the pause function, the following effects on the printing substrate web 3 are thus to be considered:                Temperature and moisture difference between the print substrate web 3 and the environmental air, with the swelling or shrinking of said print substrate web 3 that are thereby incurred with this.        Temperature difference of the print heads 5 relative to the print substrate web 3, with the shrinking of said print substrate web 3 that is incurred with this.        